The biggest and most comprehensive study ever among nuclear power workers has established that the low doses of radiation they receive can increase their cancer risk. The study of 400,000 workers in 15 countries including the UK, carried out by the International Agency for Research on Cancer (IARC), an arm of the World Health Organisation, found that 1-2% of the cancer deaths among those who took part may have been caused by radiation exposure. The findings will be controversial, because the nuclear industry has never accepted that its workers are at increased risk.

The research is overdue. The standards used for assessing acceptable levels of radiation exposure are currently based on studies of the survivors of the atomic bombing of Nagasaki and Hiroshima in August 1945. This told scientists much about sudden exposure to high levels of radiation, but it did not give a definitive picture of the effects of low-level long-term exposure.

"There has been a controversy for decades about the use of data on A-bomb survivors for setting standards for the protection of the general public and radiation workers," said Elisabeth Cardis of the IARC radiation group. "There was therefore a need for a direct assessment of the carcinogenic effects of low-dose exposures, to evaluate the adequacy of these standards."

The IARC scientists looked back at the exposure levels over one year of 400,000 workers, 90% of them men. The workers had all worn radiation dosimeters which registered their exposure. Some worked in nuclear power plants, some in nuclear research or waste management and others in the production of nuclear fuel, isotopes or weapons. Those who might have had substantial neutron or plutonium exposure were excluded, because these have not been adequately measured in the past, said the IARC. Of the 400,000, there had been 196 deaths from leukaemia (other than chronic lymphocytic leukaemia) and 6,519 deaths from other cancers. The risk of cancers in these groups forms the basis for radiological protection standards.

The IARC said: "Many of the subjects in this study worked in the early years of the industry when doses tended to be higher than they are today, however. Only a small proportion of cancer deaths would therefore be expected to occur from low-dose chronic exposures to X- and gamma-radiation among current nuclear workers and in the general population." The extra risk may be small, but the scientists say that although it may be made worse by smoking, cigarettes would not account for all of it. "These results provide the most precise and comprehensive direct estimates of cancer risk after protracted exposure to low doses of ionising radiation," said Peter Boyle, director of the IARC. "They strengthen the scientific basis of radiation protection standards for environmental, occupational, and medical diagnostic exposures."